Exposure tools are used in photolithography to define patterns on objects such as semiconductor wafers. Exposure tools, however, may introduce errors that cause the resulting pattern defined on the object to differ from the intended pattern. Known techniques may generate optical models that may be used to compensate for errors introduced by exposure tools. The optical models generated by known techniques, however, may fail to provide information about certain errors. Consequently, known techniques for generating optical models may be unsatisfactory in certain situations.